Giant subduction megathrust earthquakes of magnitude 9 and larger pose a significant tsunami hazard in coastal regions. In order to test and improve empirical tsunami forecast models and to explore the susceptibility of different subduction settings we here analyze the scaling of subduction earthquake‐triggered tsunamis in the near field and their variability related to source heterogeneities. We base our analysis on a sequence of 50 experimentally simulated great to giant ( M w = 8.3–9.4) subduction megathrust earthquakes generated using an elastoplastic analog model. Experimentally observed surface deformation is translated to local tsunami runup using linear wave theory. We find that the intrinsic scaling of local tsunami runup is characterized by a linear relationship to peak earthquake slip, an exponential relationship to moment magnitude, and an inverse power law relationship to fore‐arc slope. Tsunami variability is controlled by coseismic slip heterogeneity and strain localization within the fore‐arc wedge and is characterized by a coefficient of variation C v ∼ 0.5. Wave breaking modifies the scaling behavior of tsunamis triggered by the largest ( M w > 8.5) events in subduction settings with shallow dipping (<1–2°) fore‐arc slopes, limits tsunami runup to <30 m, and reduces its variability to C v ∼ 0.2. The resulting effective scaling relationships are validated against historical events and numerical simulations and reproduce empirical scaling relationships. The latter appear as robust and liberal estimates of runup up to magnitude M w = 9.5. A global assessment of tsunami susceptibility suggests that accretionary plate margins are more prone to tsunami hazard than erosive margins.
The Pacific Ocean is surrounded by subduction zone systems leading to a decreasing surface area as well as sub-surface mantle domain. In contrast, the Atlantic realm is characterized by passive margins and growing in size. To maintain global mass balance, the Caribbean and the Scotia Sea have been proposed as Pacific-to-Atlantic transfer channels for sub-lithospheric shallow mantle. We concentrate on the Caribbean here and test this idea by calculating the present-day regional dynamic topography in search of a gradual decrease from west to east that mirrors the pressure gradient due to the shrinkage of the Pacific. To calculate the dynamic topography, we isostatically correct the observed topography for sediments and crustal thickness variations, and compare the result with those predicted by lithospheric cooling models. The required age-grid was derived from our recently published reconstruction model. Our results confirm previous geochemical and shear-wave splitting studies and suggest some lateral asthenosphere flow away from the Galapagos hotspot. However, they also indicate that this flow is blocked in the Central Caribbean. This observation suggests that rather than through large scale Pacific-to-Atlantic shallow mantle flow, the global mass balance is maintained through some other process, possibly related to the deep mantle underneath Africa.
A striking feature of the Indian Ocean is a distinct geoid low south of India, pointing to a regionally anomalous mantle density structure. Equally prominent are rapid plate convergence rate variations between India and SE Asia, particularly in Late Cretaceous/Paleocene times. Both observations are linked to the central Neo-Tethys Ocean subduction history, for which competing scenarios have been proposed. Here we evaluate three alternative reconstructions by assimilating their associated time-dependent velocity fields in global high-resolution geodynamic Earth models, allowing us to predict the resulting seismic mantle heterogeneity and geoid signal. Our analysis reveals that a geoid low similar to the one observed develops naturally when a long-lived back-arc basin south of Eurasia's paleo-margin is assumed. A quantitative comparison to seismic tomography further supports this model. In contrast, reconstructions assuming a single northward dipping subduction zone along Eurasia's margin or models incorporating a temporary southward dipping intra-oceanic subduction zone cannot sufficiently reproduce geoid and seismic observations.
Most authors agree that parts of the Caribbean plate are an igneous Plateau underlain by Farallon lithosphere that was trapped in between the North and South American plates. However, the origin of the thickened crust is debated. The theory of oceanic plateaus forming as magmatic outpouring related to a plume arrival became prominent when Large Igneous Provinces could be traced back to hotspots. The present-day proximity of the Galapagos hotspot made it an obvious candidate for associating its plume head arrival with the formation of the Caribbean Plateau. However, it was shown that in a fixed or moving Indian-Atlantic hotspot reference frame, plate reconstructions predicted the Galapagos hotspot a thousand or more kilometres away from the Caribbean plate at the time of Plateau formation (∼88–94 Ma). Here, we calculate the goodness of fit for the Pacific hotspot reference frame and the recently developed Global Moving Hotspot Reference Frame. We show that both frames lead to good correlations between the paleo-positions of the Caribbean Plate and the Galapagos hotspot, when a docking time of the Caribbean plate to South America of 54.5 Ma is assumed. As this result is consistent with abundant evidence that lends support for a Galapagos hotspot origin of the rocks that form the Caribbean Plateau, proposed alternative mechanisms to explain the thickened crust of the Caribbean Plateau seem to be unnecessary. Finally, based on our model, we also derived an age distribution of the lithosphere underneath the thickened crust of the Caribbean Plateau.
The Scotia Sea Region in the South Atlantic is a complex tectonic area, mainly characterized by back-arc spreading processes active on various time scales. It has played a key role for the global climate, as the opening of Drake Passage possibly led to the onset of the Antarctic Circumpolar Current (ACC). Unfortunately, geophysical data for this region is relatively sparse and plate reconstructions suffer from disagreement about the ages of much of the seafloor in the region. In particular, the ambiguity dating magnetic lineations for the Protector, Dove, and Discovery basin allows for several scenarios for the Scotia Sea to form. Other great uncertainty lies in the central Scotia Sea: here models suggest it could either be a Mesozoic South American plate fragment or a Miocene back-arc basin. Furthermore, the nature of Discovery Bank, an elevated area in the southeast of the Scotia Sea, has remained controversial. It has been interpreted as a remnant arc of a former subduction zone in the northern Weddell Sea, or as stretched continental fragment probably originating from the tip of South America. We present these scenarios using digital reconstructions of the different lithospheric components and calculate the dynamic topography of each basin using sediment thickness estimations from literature, including error estimation, where available. Our results show mostly isostatically compensated crust or little dynamic topography of up to 500 m for the former Phoenix Plate and Western Scotia Sea, respectively. The central Scotia Sea, if a Miocene age is assumed, shows a very similar appearance with respect to its westerly neighbors. The idea of a Mesozoic central Scotia Sea plate fragment, on the other hand, leads to a strong positive dynamic topography signal of more than 1.5 km. As for the East Scotia Sea, negative dynamic topography of approximately 1 km was calculated. Furthermore, we observed a trend in the dynamic topography, that is a slight increase from north to south. These results may indicate a lateral mantle flow from underneath West Scotia, around the northern rim of the central Scotia and as far as East Scotia, where the negative dynamic topography may be related to overriding of the Sandwich trench slab. The relative high dynamic topography signal in the basins of southern Scotia Sea may relate to the hypothesized remnant arc, the Discovery Bank, and may be an explanation for potential blockage of lateral mantle flow in this region.
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